Clinical and Morphological Changes Following 2 Rehabilitation Programs for Acute Hamstring Strain Injuries: A Randomized Clinical Trial

Abstract

Study design:
Randomized, double-blind, parallel-group clinical trial.

Objectives:
To assess differences between a progressive agility and trunk stabilization rehabilitation program and a progressive running and eccentric strengthening rehabilitation program in recovery characteristics following an acute hamstring injury, as measured via physical examination and magnetic resonance imaging (MRI).

Background:
Determining the type of rehabilitation program that most effectively promotes muscle and functional recovery is essential to minimize reinjury risk and to optimize athlete performance.

Methods:
Individuals who sustained a recent hamstring strain injury were randomly assigned to 1 of 2 rehabilitation programs: (1) progressive agility and trunk stabilization or (2) progressive running and eccentric strengthening. MRI and physical examinations were conducted before and after completion of rehabilitation.

Results:
Thirty-one subjects were enrolled, 29 began rehabilitation, and 25 completed rehabilitation. There were few differences in clinical or morphological outcome measures between rehabilitation groups across time, and reinjury rates were low for both rehabilitation groups after return to sport (4 of 29 subjects had reinjuries). Greater craniocaudal length of injury, as measured on MRI before the start of rehabilitation, was positively correlated with longer return-to-sport time. At the time of return to sport, although all subjects showed a near-complete resolution of pain and return of muscle strength, no subject showed complete resolution of injury as assessed on MRI.

Conclusion:
The 2 rehabilitation programs employed in this study yielded similar results with respect to hamstring muscle recovery and function at the time of return to sport. Evidence of continuing muscular healing is present after completion of rehabilitation, despite the appearance of normal physical strength and function on clinical examination.

Level of evidence:
Therapy, level 1b-.

Full text

Clinical and Morphological Changes Following 2 Rehabilitation
Programs for Acute Hamstring Strain Injuries: A Randomized
Clinical Trial
AMY SILDER, PhD1, MARC A. SHERRY, PT, DPT, LAT, CSCS2, JENNIFER SANFILIPPO,
MS, LAT3, MICHAEL J. TUITE, MD4, SCOTT J. HETZEL, MS5, and BRYAN C.
HEIDERSCHEIT, PT, PhD6
1Department of Bioengineering and Department of Orthopaedic Surgery, Stanford University,
Stanford, CA
2Sports Rehabilitation, University of Wisconsin Health Sports Medicine, Madison, WI
3Athletics Department, University of Wisconsin-Madison, Madison, WI
4Department of Radiology, University of Wisconsin-Madison, Madison, WI
5Department of Biostatistics and Medical Informatics, University of Wisconsin-Madison, Madison,
WI
6Department of Orthopedics and Rehabilitation and Department of Biomedical Engineering,
University of Wisconsin-Madison, Madison, WI
Abstract
STUDY DESIGN—Randomized, double-blind, parallel-group clinical trial.
OBJECTIVES—To assess differences between a progressive agility and trunk stabilization
rehabilitation program and a progressive running and eccentric strengthening rehabilitation
program in recovery characteristics following an acute hamstring injury, as measured via physical
examination and magnetic resonance imaging (MRI).
BACKGROUND—Determining the type of rehabilitation program that most effectively promotes
muscle and functional recovery is essential to minimize reinjury risk and to optimize athlete
performance.
METHODS—Individuals who sustained a recent hamstring strain injury were randomly assigned
to 1 of 2 rehabilitation programs: (1) progressive agility and trunk stabilization or (2) progressive
running and eccentric strengthening. MRI and physical examinations were conducted before and
after completion of rehabilitation.
RESULTS—Thirty-one subjects were enrolled, 29 began rehabilitation, and 25 completed
rehabilitation. There were few differences in clinical or morphological outcome measures between
rehabilitation groups across time, and reinjury rates were low for both rehabilitation groups after
return to sport (4 of 29 subjects had reinjuries). Greater craniocaudal length of injury, as measured
on MRI before the start of rehabilitation, was positively correlated with longer return-to-sport
Copyright ©2013 Journal of Orthopaedic & Sports Physical Therapy®
Address correspondence to Dr Marc A. Sherry, University of Wisconsin Sports Medicine Center, 621 Science Drive, Madison, WI
53711. MSherry@UWHealth.org.
The authors certify that they have no affiliations with or financial involvement in any organization or entity with a direct financial
interest in the subject matter or materials discussed in the manuscript. This study was approved by the University of Wisconsin Health
Sciences Institutional Review Boards.
NIH Public Access
Author Manuscript
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
Published in final edited form as:
J Orthop Sports Phys Ther
. 2013 May ; 43(5): 284–299. doi:10.2519/jospt.2013.4452.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

time. At the time of return to sport, although all subjects showed a near-complete resolution of
pain and return of muscle strength, no subject showed complete resolution of injury as assessed on
MRI.
CONCLUSION—The 2 rehabilitation programs employed in this study yielded similar results
with respect to hamstring muscle recovery and function at the time of return to sport. Evidence of
continuing muscular healing is present after completion of rehabilitation, despite the appearance of
normal physical strength and function on clinical examination.
LEVEL OF EVIDENCE—Therapy, level 1b–.
J Orthop Sports Phys Ther 2013;43(5):284-299.
Epub 13 March 2013. doi:10.2519/jospt.2013.4452
Keywords
MRI; muscle; return-to-sport criteria
Acute hamstring strain injuries are common in sports involving high-speed
movements.7,11,14,24,32 Many athletes return to sport at a suboptimal level of performance,32
which may contribute to high reinjury rates reported to vary from approximately
15%11,12,35,36 to more than 50%.3,21 This has led to speculation that inadequate
rehabilitation and/or a premature return to sport may be to blame.21,24,31 Determining the
type of rehabilitation program that most effectively promotes muscle tissue and functional
recovery is essential to minimize the risk of reinjury and to optimize athlete performance.
Neuromuscular control exercises9,23 and eccentric training1,2,7,13,25,28 have been shown to
reduce the likelihood of hamstring injury and are advocated by many to be included as part
of rehabilitation following an acute strain injury. Eccentric strengthening, in particular, is
believed to increase the series compliance of muscle and allow for longer operating
lengths,8,26 which may offset the effects of scar tissue.27 Alternatively, Sherry and Best30
found significantly lower reinjury rates in athletes who completed a progressive agility and
trunk stabilization (PATS) program, compared to those whose rehabilitation programs
focused on isolated hamstring strengthening and stretching. The authors speculated that the
inclusion of exercises targeting muscles that control pelvic motion early in the rehabilitation
process might have facilitated recovery from injury and thereby minimized reinjury risk.
While both the PATS and the eccentric strengthening rehabilitation programs are promising
and may be effective, they have not been directly compared with regard to restoring muscle
integrity and function.
It is possible that, regardless of the rehabilitation employed, clinical determinants of
recovery, as measured during physical exam (eg, no pain, full range of motion, and full
strength), do not adequately represent complete muscle recovery and readiness to return to
sport. Despite meeting clinical clearance, 37% of the athletes in a study by Connell et al,10
as assessed with magnetic resonance imaging (MRI), showed continued evidence of muscle
healing after returning to sport, suggesting that athletes may be in an injury-susceptible
state.4,10,29,31,34 The use of MRI near the time of injury has an established prognostic role in
estimating convalescent period. A greater amount of T2 hyperintensity, reflective of edema,
is associated with a longer rehabilitation time. This correlation has been made using
measurements of cranio-caudal (CC) injury length,10,29,34 percent cross-sectional area of
injury,10,31 distance of maximum signal intensity from the ischial tuberosity,4 and maximum
T2 hyperintensity.10,31 Regardless of the rehabilitation employed, determining the extent of
remaining injury on MRI using these same metrics following the completion of a
rehabilitation program may yield further insights into the readiness of the athlete to return to
sport.
SILDER et al. Page 2
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

The purpose of this study was to monitor clinical and morphological changes during the
course of rehabilitation in individuals with acute hamstring strain injuries and to determine if
differences in outcomes may exist between the 2 progressive rehabilitation programs. The
rehabilitation programs utilized were a modified PATS program30 and a progressive running
and eccentric strengthening (PRES) program. We hypothesized that athletes participating in
the PATS program would display a greater amount of muscle recovery at the time of return
to sport compared to those in the PRES group. We further hypothesized that, regardless of
the rehabilitation employed, the majority of athletes would display continued signs of
healing on MRI after being clinically cleared to return to sport. Further analyses of time
needed to return to sport and MRI measurements were performed to more fully characterize
the timeline of hamstring muscle recovery following injury.
METHODS
Trial Design and Participants
This was an equal-randomized, double-blind, parallel-group study. Potential subjects were
identified and recruited via physicians, athletic trainers, and physical therapists in Madison,
WI and the surrounding communities over a 3-year period. To be eligible for enrollment,
individuals had to present with a suspected hamstring injury occurring within the prior 10
days, to be 16 to 50 years of age, and to be involved in sports that require high-speed
running (eg, football) a minimum of 3 days per week. All subjects or parents/guardians
provided informed consent to participate in this study, according to a protocol approved by
the University of Wisconsin Health Sciences Institutional Review Boards. All testing took
place at the University of Wisconsin Hospital and Clinics.
All enrolled subjects received a physical examination and MRI within 10 days of the injury.
Hamstring injury was confirmed by physical examination conducted by a physical therapist
(B.C.H.) and was based on a sudden-onset mechanism and the presence of 2 or more of the
following: palpable pain along any of the hamstring muscles, posterior thigh pain without
radicular symptoms during a passive straight leg raise, weakness with resisted knee flexion,
pain with resisted knee flexion, and/or posterior thigh pain with sports/running. Subjects
were excluded from this study if they were identified as having a complete hamstring
disruption or avulsion during the initial physical examination or MRI.
Randomization
Following the initial physical examination, the treating physical therapist (M.A.S.) used a 4-
block, fixed-allocation randomization process to assign subjects to 1 of the 2 rehabilitation
groups (the PATS or PRES group). This randomization process allowed stratification for
age, initial injury or recurrent injury, and mechanism of injury. These variables have
previously been shown to affect return-to-sport time and reinjury rates.3,7,15-17 The random
allocation sequence was generated by an independent biostatistician.
Interventions
Each subject completed rehabilitation with the same physical therapist (M.A.S.), who was
blinded to any information obtained from the initial physical examination and MRI. Each
rehabilitation program had 3 treatment phases. In the first phase, ice was applied to the
posterior thigh for 20 minutes after completing each rehabilitation session. Subjects
progressed into phase 2 when they could walk with the same stride length and stance time
on the injured and non-injured limbs (visually assessed) and initiate a pain-free isometric
hamstring contraction at 90° of knee flexion with a manual muscle testing grade judged to
be at least 4/5. Subjects progressed into phase 3 when they could jog forward and backward
with the same stride length and stance time on the injured and noninjured limbs (visually
SILDER et al. Page 3
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

assessed) and demonstrate 5/5 strength on manual muscle testing of the hamstrings in 3
conditions: prone at 90° of knee flexion with the tibia in neutral position, the tibia rotated
internally, and the tibia rotated externally.
The PATS group participated in a modified version of the original PATS rehabilitation
program.30 The original PATS program was modified from 2 phases to 3 phases, which
allowed for more progressive resistance during the trunk stabilization exercises and added a
lunge walk that required trunk rotation and pelvic control with the hamstrings in a
lengthened position (APPENDIX A). The progressive agility exercises began with
movements primarily in the frontal and transverse planes during phase 1 and progressed to
agility and trunk stabilization movements in the transverse and sagittal planes during phase
2. Phase 3 increased the speed and/or resistance of the exercises.
The PRES group performed a rehabilitation program consisting of progressive running and
eccentric strengthening that was modeled after the work of Baquie and Reid6(APPENDIX
B). Phase 1 consisted of a short-stride jog and hamstring isometric exercises. Phase 2
incorporated concentric and eccentric strengthening exercises, and phase 3 progressed to
intense eccentric strengthening with a power component. Running during phases 2 and 3
consisted of performing a series of sprints with progressive acceleration/deceleration
(APPENDIX C).
Treatment implementation and return-to-sport criteria were the same for both rehabilitation
groups. Rehabilitation was to be completed 5 days per week at home. Subjects were asked to
track their compliance on an exercise log that was submitted at each follow-up visit. Follow-
up visits were scheduled according to patient progress and reported symptoms, and
participants were monitored by phone calls or electronic mail every few days. A minimum
of 1 weekly clinic visit was required of all subjects to monitor exercise technique and to re-
evaluate their status. Subjects were allowed to return to sport when they had no palpable
tenderness along the posterior thigh, demonstrated subjective readiness (no apprehension)
after completing a series of progressive sprints working up to full speed, and scored 5/5 on
manual muscle testing of the hamstrings performed on 4 consecutive repetitions in various
knee positions. Knee flexion isometric strength testing was performed in prone with the hip
in 0° of flexion and the knee flexed at 90° and 15°. Testing was performed with the tibia in
neutral, external rotation, and internal rotation for both knee flexion angles. After being
cleared to return to sport by the treating physical therapist, all subjects received a final
physical exam and MRI. Any subject who incurred a re-injury at any time during
rehabilitation or the 6 months following return to sport received a follow-up MRI as soon as
possible after the reinjury and, at that point, discontinued study participation.
Outcomes
Primary Outcome Measures—The primary outcome measure was return-to-sport time
(days), defined as the period from initial injury to completion of rehabilitation. The CC
length of injury, as measured on MRI, was also of primary interest and was measured as the
total injured area, accounting for the likelihood that more than 1 muscle would show signs
of injury.10,31,33 All MRI studies were conducted using a phased-array torso coil in a 1.5-T
TwinSpeed scanner (GE Healthcare, Waukesha, WI). T2-weighted axial and coronal images
were obtained using the following scan parameters: TR/TEeff, 2200 to 3200 divided by 70 to
88 milliseconds; matrix, 512 × 512; 1 NEX; 5-mm axial with no gap; and 4.0/0.4-mm
coronal. Images were interpreted by the same musculoskeletal radiologist (M.J.T.), who was
unaware of rehabilitation group allocation or clinical details other than suspected hamstring
injury. Each image set was examined separately to help ensure unbiased measurements.
SILDER et al. Page 4
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

Secondary Outcome Measures—Mediolateral width and anterior/posterior depth of the
total injured area were also measured on MRI. The cross-sectional area (0.25 × π ×
mediolateral × anterior/posterior) of the injury, as a percentage of the total cross-sectional
area, was calculated at the level where the injury had the largest absolute cross-sectional
distribution in the muscle(s) (FIGURE 1).5,10,29,31,34 In addition, the axial slice on the initial
examination with the brightest signal intensity was used to measure maximum T2
hyperintensity. On the final MRI, T2 hyperintensity was measured at the corresponding
anatomical location. To account for variations in signal quality between examinations, these
values were normalized to the average signal intensity in normal, uninjured muscle tissue at
their respective time points. Finally, the site of injury was categorized as having occurred to
the biceps femoris, semimembranosus, or semitendinosus, as well as having occurred in
either the tendon or the proximal, middle, or distal musculotendon junction. Note that no
subject in this study experienced an injury to the distal aspect of any of the hamstring
muscles.
Both physical examinations were conducted by the same physical therapist (B.C.H.), who
was unaware of the type of rehabilitation employed or any information obtained from MRI.
The subjects’ use of ice and nonsteroidal anti-inflammatory drugs (NSAIDs) prior to
enrollment was noted, and all subjects were asked to refrain from NSAIDs once enrolled.
The physical examination included bilateral measures of range of motion, strength, and both
location and distribution (length) of pain. Surface palpation was used to determine the
location of maximal tenderness, which was measured (cm) relative to the ischial tuberosity.
The total CC length (cm) of pain in the muscle/tendon unit was also measured with
palpation. The passive straight leg raise was performed with the knee in full extension,
whereas active and passive knee extension was performed with the hip in 90° of flexion, and
joint angles were recorded at the instant of initial hamstring discomfort/pain on the injured
side. Isometric knee flexion strength was measured with the subject prone and the knee
flexed to 90° and 15°. When the knee was flexed to 90°, knee flexion strength was also
measured with the lower leg in neutral, internal rotation, and external rotation. Isometric hip
extension strength was measured with the knee at 0° and 90° of flexion. Pain provocation
was noted for all strength tests, with strength recorded using a standard manual muscle
testing grading scale. As part of the physical examination performed at the time of return to
sport, subjects were asked (yes/no) if they (1) were back to their preinjury level of
performance, and, if not, whether the hamstring injury was a limiting factor, (2) had any
remaining symptoms, and (3) felt hamstring symptoms during running.
After returning to sport, reinjury occurrence was monitored by phone calls or electronic mail
at 2 weeks and at 3, 6, 9, and 12 months. A subject was considered to have a reinjury if there
was a specific mechanism that caused a return of posterior thigh pain, pain with resisted
knee flexion, tenderness to palpation along the muscle/tendon unit, and decreased ability to
do sporting activities (perceived strength and power).
Statistical Analysis
A priori sample-size calculation, based on time to return to sport, was performed under the
assumption that the standard deviation of time to return to sport would be equal to the
difference in time to return to sport between the 2 rehabilitation programs. To achieve 80%
power for a
t
test under these assumptions, it was necessary to include 17 subjects per group.
All data were analyzed based on intention to treat. Missing data were treated as missing at
random. Subjects who sustained a reinjury were documented, and reinjury rates were
compared between groups. The data of subjects who sustained a reinjury were included in
the analysis up to the time of reinjury and considered as missing after the reinjury, so as not
SILDER et al. Page 5
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

to skew their rehabilitation results. This method should not have greatly affected the results,
because reinjury rates were uncommon and similar between the groups.
Analysis of subject baseline characteristics between the 2 randomly assigned rehabilitation
groups was conducted using
t
tests or Wilcoxon rank-sum tests for nonnormally distributed
data and the Fisher exact test for categorical characteristics. Analysis of time to return to
sport was performed with a 2-sample
t
test. Analyses of change in variables over time were
examined with repeated-measures analyses of variance, with time, intervention group, and
their interaction as fixed effects and subject as a random effect. The repeated-measures
analyses of variance were used to estimate the mean and 95% confidence interval (CI) at
each of the time points. Analyses of the association of categorical outcomes and program
assignment were conducted with Fisher exact tests. The correlation between time to return to
sport and CC length of injury per MRI measure was calculated with a Pearson correlation
coefficient. All tests were 2 sided, and significance was set at
α
= .05.
RESULTS
Of the 31 subjects enrolled, 1 subject was excluded because of a biceps femoris avulsion
identified on initial MRI, and 1 subject was excluded due to sacroiliac pathology with
referred posterior thigh pain (FIGURE 2). Twenty-nine subjects began rehabilitation. Two
of those subjects dropped out of the study without reinjury prior to completion of
rehabilitation. In addition, 2 subjects sustained a reinjury during the course of rehabilitation.
One reinjury occurred during the sprinting portion of return-to-sport testing (subject 26,
PRES group). The other reinjury occurred during phase 3 of the PATS program, while
performing a single-leg chair bridge (subject 27). A total of 25 subjects completed
rehabilitation; however, only 24 subjects (19 male, 5 female; mean ± SD age, 24 ± 9 years;
height, 1.80 ± 0.09 m; weight, 79 ± 15 kg) completed return-to-sport testing, because subject
3 sustained a re-injury on the same day he was cleared to return to sport but prior to his
scheduled return-to-sport testing.
Initial MRI
The time of initial MRI relative to the time of injury occurred later in the PRES group, with
a median (interquartile range [IQR]) of 7 (6-7) days after injury, compared to 5 (3-6) days in
the PATS group (
P
= .041). With respect to which muscles were determined as being
injured, the MRI and physical examinations agreed in all but 9 of the 29 initial cases; 3
subjects showed no abnormal T2 intensity on initial MRI, and 6 showed disagreement
between the clinical and MRI diagnoses as to the primary muscle injured (TABLE 1).
The following results consider only the 26 subjects with MRI indication of injury (T2
hyperintensity). Injury was isolated to only 1 muscle in 12 subjects, visible in 2 muscles for
10 subjects, visible in 3 muscles for 3 subjects, and visible as T2 hyperintensity in 4 muscles
for 1 subject (group difference,
P
= .180) (TABLE 1). The median (IQR) initial percent
cross-sectional area injured, when considering all muscles involved, was 63% (36%-79%) in
the PATS group and 61% (48%-91%) in the PRES group (
P
= .233), and the mean ± SD
maximum T2 signal intensity was 3.1 ± 1.0 times that of the uninjured muscle in the PATS
group and 2.8 ± 0.7 times that of the uninjured muscle in the PRES group (
P
= .518)
(TABLE 2). No significant differences between rehabilitation groups were found for any of
the initial MRI measurements.
Initial Physical Examination
The initial physical examination occurred a median (IQR) of 4 (3-6) days after injury in the
PATS group and 6 (4-7) days after injury in the PRES group (
P
= .161). Subject questioning
SILDER et al. Page 6
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

revealed that 17 of the 29 subjects (9 of 16 in the PATS group and 8 of 13 in the PRES
group) took NSAIDs within 1 to 3 days after the injury and that 7 subjects (3 in the PATS
group) continued NSAID use until enrollment in this study. All of the subjects reported
using ice within 1 to 3 days after injury, and 18 (8 in the PATS group) continued icing
through enrollment in this study. The median (IQR) distance of maximum pain during
palpation was 7.4 cm (0.0-16.1) distal to the ischial tuberosity in the PATS group and 7.1
cm (5.5-9.3) in the PRES group (
P
= .961). The mean ± SD length of pain with palpation
was 9.9 ± 5.2 cm and 8.3 ± 3.0 cm in the PATS and PRES groups, respectively (
P
= .507).
Manual strength testing revealed that not all of the subjects exhibited strength deficits on
their injured limb during all tests; however, every subject showed a strength deficit during at
least 1 strength test (TABLE 3). Range-of-motion tests revealed that some of the subjects
exhibited greater range of motion in their injured limb compared to the uninjured limb. No
significant differences between rehabilitation groups were found for any of the initial
physical examination measurements.
Primary Outcome Measures
The mean ± SD time to return to sport was 28.8 ± 11.4 days in the PRES rehabilitation
group and 25.2 ± 6.3 days in the PATS rehabilitation group (
P
= .346). The mean CC length
of injury from the initial MRI examination was 12.8 cm (95% CI: 7.7, 18.0) in the PATS
group and 17.3 cm (95% CI: 9.8, 24.7) in the PRES group (
P
= .229). Initial CC length of
injury was significantly associated with a longer return-to-sport time (
r
= 0.41,
P
= .040). At
return to sport, CC length in the PRES group was 15.9 cm (95% CI: 8.4, 23.4) compared to
7.9 cm (95% CI: 2.7, 13.1) in the PRES group (
P
= .037). The subjects in the PRES group
also displayed less improvement in injury length, with an average improvement from
baseline of 1.4 cm (95% CI: –1.9, 4.7) compared to 5.0 cm (95% CI: 2.7, 7.2) for those in
the PATS group (
P
= .035). Ede-ma and hemorrhage can extend into the fascial plane, which
can lengthen the CC extent of injury over time (FIGURE 3). As a result, the change in CC
injury length over the course of rehabilitation was variable among all subjects, ranging from
a 137% increase in length (subject 22) to a 100% decrease in length. The mean ± SD
improvement of only those subjects with MRI indication of injury who completed all
rehabilitation and testing (24 subjects) was 39% ± 35% (TABLE 2).
Secondary Outcome Measures
Rehabilitation—The median (IQR) number of days until return to sport was 23 (21-28)
and 28 (20-33) in the PATS and PRES groups, respectively (
P
= .512). The median (IQR)
number of clinic visits was 4 (3-5) in both groups, and subjects completed a median (IQR)
of 20 (13-21) days of rehabilitation at home in the PATS group and 21 (13-28) days in the
PRES group (
P
= .577). Based on self-reported exercise logs, rehabilitation compliance was
slightly but not significantly higher in the PRES group (mean ± SD, 88% ± 9%) than in the
PATS group (80% ± 12%,
P
= .070). No significant differences in return-to-sport time,
clinic visits, or rehabilitation compliance were noted between rehabilitation groups.
Final MRI—No subject showed complete injury resolution (no T2 hyperintensity) after
being cleared to return to sport (TABLE 2). The mean percent cross-sectional area injured,
when considering all muscles involved, was 45.0% (95% CI: 28.9%, 61.1%) at baseline in
the PATS group and 61.9% (95% CI: 38.8%, 85.1%) at baseline in the PRES group (
P
= .
145). The PATS group improved to a remaining mean percent cross-sectional injured area of
19.2% (95% CI: 2.6%, 35.8%) at follow-up, compared to 33.3% (95% CI: 9.0%, 57.7%) in
the PRES group (
P
= .244). The mean improvement from baseline in percent cross-sectional
area injured was 25.8% (95% CI: 8.3%, 43.3%) in the PATS group, compared to 28.6%
(95% CI: 9.8%, 47.4%) in the PRES group (
P
= .822). The mean normalized T2 signal
intensity decreased from baseline slightly more in the PATS group (–0.75; 95% CI: –1.2, –
SILDER et al. Page 7
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

0.31) compared to the PRES group (–0.50; 95% CI: –0.98, –0.03), but this difference was
not significant (
P
= .438). Finally, the presence of early scar tissue formation was apparent
in many of the subjects (FIGURES 3 and 4).
Final Physical Examination—Eleven subjects (7 of 13 remaining subjects in the PATS
group and 4 of 12 remaining subjects in the PRES group) indicated that they felt remaining
hamstring symptoms (eg, pain, tightness) after being cleared to return to sport (
P
= .444).
Twelve subjects (7 in the PATS group and 5 in the PRES group) indicated that they did not
feel that they had returned to their preinjury level of performance (
P
= 1.0). However, only 3
subjects (2 in the PATS group and 1 in the PRES group) reported that their hamstring injury
was a limiting factor in their performance, and general deconditioning was the most cited
limiting factor. Pain with palpation and during manual strength tests was nearly absent for
all subjects at the time of return to sport (TABLE 3). The subjects in the PRES group
showed greater range of motion during the straight leg raise in the noninjured limb at the
final physical exam, as opposed to those in the PATS group, who exhibited greater range of
motion in the injured limb. Additionally, the subjects in the PRES group tended to show
greater mean side-to-side difference in the straight leg raise (noninjured limb – injured limb)
at the final physical examination (3.4°; 95% CI: –4.0°, 10.7°) compared to those in the
PATS group (–1.8°; 95% CI: –9.7°, 6.2°), but that difference was not significant (
P
= .337).
This trend in the magnitude of the side-to-side difference between groups was consistent
with the findings at baseline, where the side-to-side difference was 18.6° (95% CI: 11.6°,
25.7°) for the PATS group and 9.4° (95% CI: 2.0°, 16.7°) for the PRES group (
P
= .074). No
significant differences between rehabilitation groups were observed during the final physical
examination or in the amount of improvement between the initial and final physical
examinations.
Symptoms and Reinjury Through 12 Months
Two of the 4 subjects who reinjured themselves did so between completion of rehabilitation
and the following 12-month period. Subject 3 (PRES group) sustained a reinjury on the
same day as being cleared to return to sport, and subject 17 (PRES group) sustained a
reinjury 4 days after completion of rehabilitation. At 2 weeks following return to sport, only
5 subjects (1 in the PATS group and 4 in the PRES group) reported continued symptoms
that limited their normal participation in sport. At approximately 6 weeks after return to
sport, subject 10 (PRES group) ruptured the anterior cruciate ligament in the contralateral
knee while landing from a jump while playing basketball, thereby limiting participation in
sport. At 3, 6, 9, and 12 months following return to sport, anywhere between 2 and 5
subjects reported continuing symptoms.
MRI of Reinjury
Of the 4 subjects who sustained a reinjury, only 3 received additional MRI. Re-injuries for
those 3 subjects occurred in generally the same location as the initial injury, and injury
severity did not appear worse than the initial injury (FIGURE 4). To help establish whether
any MRI measurement could be a predictor of reinjury, post hoc analysis was conducted to
compare the extent of muscle damage measured on initial MRI between the 4 subjects who
were reinjured and the other 25 subjects. The reinjured subjects had a significantly greater
percent area injured on initial MRI (4 reinjured subjects, 87% [95% CI: 68%, 100%]; the
remaining 25 subjects, 54% [95% CI: 43%, 65%];
P
= .015). CC length and normalized T2
hyperintensity were not significantly different between the 4 subjects who reinjured
themselves and the remainder of subjects.
SILDER et al. Page 8
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

DISCUSSION
The purpose of this study was to compare clinical and morphological recovery
characteristics between 2 progressive rehabilitation programs for an acute hamstring strain
injury. Despite all subjects achieving a nearly complete resolution of pain and return of
isometric muscle strength on physical examination following completion of rehabilitation
(TABLE 3), no subjects exhibited complete resolution of injury on MRI (TABLE 2), and
early signs of scar tissue formation were apparent for most subjects (FIGURES 3 and 4).
Contrary to our first hypothesis, there were few differences between rehabilitation groups
with respect to muscle recovery and function. Most notably, return-to-sport times were
similar between groups, and overall reinjury rates were low (1 of 16 subjects in the PATS
group and 3 of 13 subjects in the PRES group).
In support of our hypothesis, the presence of injury on MRI was not resolved when subjects
returned to sport. Throughout the course of rehabilitation, the size of injury increased for
some subjects in terms of both CC length and cross-sectional area (TABLE 2). Cross-
sectional area increased as a result of a more diffuse but larger distribution of T2
hyperintensity. At the time of return to sport, the CC length of injury was longer for the
PRES group compared to the PATS group. Nevertheless, few clinical conclusions can be
drawn from this result, because edema drainage into the fascial plane may occur during the
course of rehabilitation and increase the apparent CC length of injury and extend the MRI
measurements beyond the actual muscle/tendon damage (FIGURE 3). Although cross-
sectional area and volume of injury are relevant indicators of damaged tissue,10,31 our
findings suggest that changes in these measures over time may not be good indicators of
injury recovery.
Through 1 year after return to sport, only 4 of the 29 subjects had sustained a reinjury, which
is a substantially lower rate than that reported by most of the previous studies.3,11,12,21,35,36
Of these 4 re-injuries, 2 occurred during rehabilitation and 2 within the first 2 weeks after
return to sport. The median return-to-sport time was 23 days, approximately 1 week longer
than other reported times.7,18 Seriousness of participation in sport may affect the
commitment of an athlete to complete rehabilitation without undue desire to return to sport
too quickly. Specifically, unlike other investigations,3,11,12,35,36 none of the subjects in this
study were professional athletes. Further, we utilized 2 of the most supported rehabilitation
programs, which is likely a key factor as to why so few subjects sustained reinjuries.
Although we observed very few differences in recovery features between rehabilitation
groups, one potential limitation of the PRES rehabilitation program is that the majority of
the rehabilitation exercises were only performed on the injured limb. This was done to
ensure the stimulus was applied to the injured leg and not compensated for by the uninjured
leg. We did not observe any clinical strength deficits at return to sport (TABLE 3) or
apprehension with sports-specific explosive movements, but it is possible that neuro-
muscular imbalances exist upon return to sport.
The CC length of injury as measured by MRI at the time of injury has been advocated as a
strong predictor of time needed to return to sport.4,10,29,31 Our results support these findings.
However, when considering the size of initial injury, past studies have considered only the
primary muscle involved when making MRI measurements.4,10,29,31 Because edema and
hemorrhage are often present in more than 1 muscle,10,31,33 we chose to estimate percent
cross-sectional area relative to all of the muscles involved in the initial injury. We believe
that this serves as a more comprehensive assessment of initial injury severity.
It is interesting to note that the 2 subjects (subjects 2 and 11) who exhibited some of the
greatest remaining muscle injury on final MRI were also the 2 subjects with the greatest
SILDER et al. Page 9
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

reported pain and strength deficits during the final physical examination. Specifically, the
CC lengths of injury for subject 2 (28.6 cm) and subject 11 (22.8 cm) were both
substantially longer than the group average (15.3 cm) of those that did not reinjure from
both groups. (TABLE 2). This finding supports the idea that edema and hemorrhage are
related to discomfort and loss of strength.19,20 Regardless, 3 subjects presented with clinical
indication of hamstring strain injury but showed no signs of T2 hyperintensity on their initial
or final MRI examinations (TABLE 2). This is not uncommon, as it has been found that 18
of 58 athletes enrolled in a previous study29 showed clinical indication of hamstring injury
but no sign of injury on MRI; 17 of these 18 athletes were classified as having a grade 1
injury. It is therefore possible that MRI evidence of injury may not be present for mild, yet
painful, hamstring injuries.
Compared to the initial injury, reinjuries within the same playing season have been shown to
occur at the same location and to be more severe on MRI.22 Based on the follow-up MRI
measures in subjects who had sustained a reinjury in this study, the reinjuries occurred in the
same location as the initial injury but were not substantially worse (FIGURE 4). It is unclear
what might have caused the contrast between these findings across the 2 studies. Post hoc
analysis indicated that the percent area injured on initial MRI in the 4 subjects who sustained
reinjuries was significantly greater than that in the subjects who were not reinjured. Percent
injured area, when including all muscles injured, may be a clinically relevant measure to aid
in determining which subjects are most at risk for reinjury; however, further study is needed
to investigate the relationship between reinjury rates and percent injured cross-sectional
area.
There are several limitations in the present study that prevented direct comparisons with the
literature and statistical conclusions and correlations between the imaging and clinical
measurements performed in this study. As some studies have done,30 we used the period
from injury to completion of rehabilitation as our definition of return-to-sport time, whereas
others have used return to competition10,29 or return to preinjury level of performance.4,5
Thus, our return-to-sport time interval (median, 23 days) was considerably less than that of
others (median, 112 days).4 A consistent limitation between our study and others10,29,34 is
the use of MRI at the time of injury. Although MRI measurements may aid the diagnosis
and treatment of hamstring strain injuries, it is not feasible for most recreational athletes to
obtain MRI following injury. Consistent with common clinical practice, we measured
strength using isometric manual muscle testing procedures. Though this measure may be
less sensitive than computerized assessments involving a dynamometer, we opted to assess
isometric strength at multiple joint positions, including short and long lengths of the
hamstring muscles. Finally, we were unable to enroll 17 subjects in each rehabilitation
group, as we initially estimated. However, our relatively small subject numbers and diverse
athletic population allowed us to present valuable data for clinicians on individual athletes,
which highlights how diversity among athletes and injury characteristics may affect
recovery during the course of rehabilitation.
CONCLUSION
In general, subjects with an acute hamstring strain injury treated with either the PATS or
PRES rehabilitation program demonstrated a similar degree of muscle recovery at the time
of return to sport. Despite this, there were no subjects who exhibited complete resolution of
injury on MRI, and 2 of the 4 subjects who reinjured themselves did so within the first 2
weeks after return to sport. It remains to be known how the gradually decreasing presence of
injury on MRI affects risk of reinjury once athletic activity is resumed. Given the results of
this study, it is important that clinicians recognize the presence of ongoing hamstring muscle
healing upon completion of a supervised rehabilitation program, despite the appearance of
SILDER et al. Page 10
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

normal strength and function on clinical examination. Based on these findings, athletes may
benefit from a gradual return to the demands of full sporting activity and from continued
independent rehabilitation after return to sport to aid in minimizing reinjury risk.
Supplementary Material
Refer to Web version on PubMed Central for supplementary material.
Acknowledgments
This study was funded by the National Football League Medical Charities, the National Institutes of Health (RR
025011), and the University of Wisconsin Sports Medicine Classic Fund. We thank Michael O’Brien and Karolyn
Davidson for their help with data analysis.
APPENDIX A
The progressive agility and trunk stabilization program consisted of 3 phases. The program
was designed to last approximately 2 to 6 weeks but progressed on a subject-specific basis,
using criteria as indicated. Intensity was used to guide the stationary biking and agility
exercises. Descriptions of the intensity levels were given to athletes and assessed
qualitatively during the activity. Low intensity was described as little to no exertion; this
intensity can be thought of as primarily used to create motion. Moderate intensity was
described as that above daily activity, with some perceived exertion. High intensity was
described as a perceived exertion near that of competitive sports.
Exercises Sets
Phase 1 Stationary bike
•Low intensity
1 × 10 min
10-m back-and-forth sidestep shuffle
•Low to moderate intensity
•Pain-free speed and stride
5 × 30 s
10-m back-and-forth grapevine
•Low to moderate intensity
•Pain-free speed and stride
5 × 30 s
Fast foot stepping in place 3 × 30 s
Prone body bridge (forearm plank) 5 × 10 s
Side body bridge (plank) 5 × 10 s on each side
Supine bent-knee bridge 10 × 5 s
Standing single-leg balance
•Progressing from eyes open to eyes closed
•Lean forward slightly
4 × 20 s for each limb
Phase 2 Stationary bike
•Moderate intensity
1 × 10 min
10-m back-and-forth sidestep shuffle 6 × 30 s
SILDER et al. Page 11
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

Exercises Sets
•Moderate to high intensity
•Pain-free speed and stride
10-m back-and-forth grapevine
•Moderate to high intensity
•Pain-free speed and stride
6 × 30 s
10-m back-and-forth boxer shuffle
•Low to moderate intensity
•Pain-free speed and stride
4 × 30 s
Rotating body bridge (hand plank)
•5-s hold on each side
2 × 10 repetitions on each
side
Supine bent-knee bridge with walk-outs
1Begin with knees very bent
2Holding hips up entire time, alternate small steps out with feet,
decreasing knee flexion
3 × 10 repetitions
Single-leg windmill touches without weight 4 × 8 repetitions per arm
per lower limb
Lunge walk with trunk rotation, opposite-hand toe touch, and T lift
•Hip flexed such that the chest and back leg are parallel to the
ground as the toe reaches to the opposite foot
2 × 10 steps per limb
Single-leg balance with forward trunk lean and opposite-leg hip extension 5 × 10 s per limb
Phase 3 Stationary bike
•Moderate to high intensity
1 × 10 min
30-m back-and-forth sideshuffle
•Moderate to high intensity
•Pain-free speed and stride
6 × 30 s
30-m back-and-forth grapevine
•Moderate to high intensity
•Pain-free speed and stride
6 × 30 s
10-m back-and-forth boxer shuffle
•Moderate to high intensity
•Pain-free speed and stride
4 × 30 s
Forward/backward accelerations
•Pain-free progression from 5 m to 10 m to 20 m
6 × 30 s
Rotating body bridge with dumbbell
•5-s hold on each side
•1.4 to 3.6 kg (3-8 lb) based on individual body weight and ability
2 × 10 repetitions
Supine single-leg chair bridge 3 × 15 repetitions
SILDER et al. Page 12
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

Exercises Sets
11 leg on a high chair with hip flexed
2Raise hips, lower, and repeat
•Progress from slow to fast speed
Single-leg windmill touches with dumbbells
•2.3 to 6.8 kg (5-15 lb) based on individual body weight and ability
4 × 8 repetitions per arm
per lower limb
Lunge walk with trunk rotation, opposite-hand toe touch, and T lift
•Hip flexed such that the chest and back lower limb are parallel to
the ground as the toe reaches to the opposite foot
•2.3 to 6.8 kg (5-15 lb) based on individual body weight and ability
Symptom-free individual practice of sport, avoiding sprinting and high-speed
maneuvers
2 × 10 steps per limb
APPENDIX B
The progressive running and eccentric strengthening program consisted of 3 phases. The
program was designed to last approximately 2 to 6 weeks but progressed on a subject-
specific basis, using criteria as indicated. Intensity was used to guide the stationary biking
and agility exercises. Descriptions of the intensity levels were given to athletes and assessed
qualitatively during the activity. Low intensity was described as little to no exertion; this
intensity can be thought of as primarily used to create motion. Moderate intensity was
described as that above daily activity, with some perceived exertion. High intensity was
described as a perceived exertion near that of competitive sports.
Exercises Sets
Phase 1 Stationary bike
•Low intensity
1 × 10 min
Increasing-effort hamstring isometrics
•Submaximal to maximal
10 × 10 s at 3 knee flexion angles
(30°, 60°, 90°)
Bilateral supine heel slides
1Lie supine on slippery surface
2Slide heels to buttock and back out
Progressive running program (APPENDIX C)
15 repetitions
Phase 2 Stationary bike
•Moderate intensity
1 × 10 min
Prone hamstring curls
•Prone with hip flexed at edge of a table (chest and stomach
on the table)
•Use ankle weights or resistance band
3 × 12 repetitions, injured limb
only
Prone hip extension off edge of bed or table through full range of
motion (chest and stomach on the table) 3 × 12 repetitions, injured limb
only
SILDER et al. Page 13
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

Exercises Sets
•Use ankle weights or resistance band
Prone leg lift and knee curl
1Lift straight leg slightly off floor (extend hip)
2Flex knee without dropping leg
Progressive running program (APPENDIX C)
2 × 12 repetitions, injured limb
only
Phase 3 Stationary bike
•Moderate to high intensity
1 × 10 min
Nordic hamstring drop-curl progression
•Complete 2 pain-free sessions before progressing to next
level
•Complete all 3 sessions, drop only, then progress through
sessions again with drop and curl
3 times per week; (1) 2 × 5 to 8
repetitions, drop only; (2) 3
× 5 to 8
repetitions, drop only; (3) 3
× 9 to 12
repetitions, drop only
Prone foot catches with ankle weight
1Lie prone with hip flexed at edge of table
2Lift leg until parallel with table
3Drop leg quickly
4Try to slow the fall and pause just before foot hits the floor
2 × 10 to 20 repetitions, injured
limb only
Prone hip extension off the edge of bed or table for full range of
motion
•Use ankle weight
1Lift leg parallel to the floor
2Drop and catch before leg touches floor
2 × 10 to 20 repetitions, injured
limb only
Standing 1-leg foot catches
1Stand against the wall
2Repeat the swing phase of sprinting, pausing just prior to
full hip flexion, with the knee extended
Symptom-free individual practice of sport, avoiding sprinting and
high-speed maneuvers
2 × 20 repetitions, injured limb
only
APPENDIX C
PROGRESSIVE RUNNING SCHEDULE
Exercises
•5 min of gentle stretching before and after each session 3 × 20 s each
- Standing calf stretch
- Standing quadriceps stretch
- Half kneeling hip flexor stretch
- Groin or adductor stretch
- Standing hamstring stretch
SILDER et al. Page 14
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

Exercises
•Repeat each level 3 times, progressing to the next level when pain free
•Maximum of 3 levels per session
•On the following session, start at the second-highest level completed
•Ice after each session, 20 min
Acceleration Distance, m Constant Speed (Maximum, 75% Speed)
Distance, m Deceleration Distance, m
Level 1 40 20 40
Level 2 35 20 35
Level 3 25 20 25
Level 4 20 20 20
Level 5 15 20 15
Level 6 10 20 10
Acceleration Distance, m Constant Speed (Maximum, 95% Speed)
Distance, m Deceleration Distance, m
Level 7 40 20 40
Level 8 35 20 35
Level 9 25 20 25
Level 10 20 20 20
Level 11 15 20 15
Level 12 10 20 10
References
1. Arnason A, Andersen TE, Holme I, Engebretsen L, Bahr R. Prevention of hamstring strains in elite
soccer: an intervention study. Scand J Med Sci Sports. 2008; 18:40–48. http://dx.doi.org/10.1111/j.
1600-0838.2006.00634.x. [PubMed: 17355322]
2. Askling C, Karlsson J, Thorstensson A. Hamstring injury occurrence in elite soccer players after
preseason strength training with eccentric overload. Scand J Med Sci Sports. 2003; 13:244–250.
[PubMed: 12859607]
3. Askling C, Saartok T, Thorstensson A. Type of acute hamstring strain affects flexibility, strength,
and time to return to pre-injury level. Br J Sports Med. 2006; 40:40–44. http://dx.doi.org/10.1136/
bjsm.2005.018879. [PubMed: 16371489]
4. Askling CM, Tengvar M, Saartok T, Thorstensson A. Acute first-time hamstring strains during
high-speed running: a longitudinal study including clinical and magnetic resonance imaging
findings. Am J Sports Med. 2007; 35:197–206. http://dx.doi.org/10.1177/0363546506294679.
[PubMed: 17170160]
5. Askling CM, Tengvar M, Saartok T, Thorstensson A. Acute first-time hamstring strains during
slow-speed stretching: clinical, magnetic resonance imaging, and recovery characteristics. Am J
Sports Med. 2007; 35:1716–1724. http://dx.doi.org/10.1177/0363546507303563. [PubMed:
17567821]
6. Baquie P, Reid G. Management of hamstring pain. Aust Fam Physician. 1999; 28:1269–1270.
[PubMed: 10650604]
7. Brooks JH, Fuller CW, Kemp SP, Reddin DB. Incidence, risk, and prevention of hamstring muscle
injuries in professional rugby union. Am J Sports Med. 2006; 34:1297–1306. http://dx.doi.org/
10.1177/0363546505286022. [PubMed: 16493170]
SILDER et al. Page 15
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

8. Butterfield TA. Eccentric exercise in vivo: strain-induced muscle damage and adaptation in a stable
system. Exerc Sport Sci Rev. 2010; 38:51–60. http://dx.doi.org/10.1097/JES.0b013e3181d496eb.
[PubMed: 20335736]
9. Cameron ML, Adams RD, Maher CG, Misson D. Effect of the HamSprint Drills training
programme on lower limb neuromuscular control in Australian football players. J Sci Med Sport.
2009; 12:24–30. http://dx.doi.org/10.1016/j.jsams.2007.09.003. [PubMed: 18077214]
10. Connell DA, Schneider-Kolsky ME, Hoving JL, et al. Longitudinal study comparing sonographic
and MRI assessments of acute and healing hamstring injuries. AJR Am J Roentgenol. 2004;
183:975–984. http://dx.doi.org/10.2214/ajr.183.4.1830975. [PubMed: 15385289]
11. Ekstrand J, Healy JC, Walden M, Lee JC, English B, Hagglund M. Hamstring muscle injuries in
professional football: the correlation of MRI findings with return to play. Br J Sports Med. 2012;
46:112–117. http://dx.doi.org/10.1136/bjsports-2011-090155. [PubMed: 22144005]
12. Elliott MC, Zarins B, Powell JW, Kenyon CD. Hamstring muscle strains in professional football
players: a 10-year review. Am J Sports Med. 2011; 39:843–850. http://dx.doi.org/
10.1177/0363546510394647. [PubMed: 21335347]
13. Engebretsen AH, Myklebust G, Holme I, Engebretsen L, Bahr R. Prevention of injuries among
male soccer players: a prospective, randomized intervention study targeting players with previous
injuries or reduced function. Am J Sports Med. 2008; 36:1052–1060. http://dx.doi.org/
10.1177/0363546508314432. [PubMed: 18390492]
14. Feeley BT, Kennelly S, Barnes RP, et al. Epidemiology of National Football League training camp
injuries from 1998 to 2007. Am J Sports Med. 2008; 36:1597–1603. http://dx.doi.org/
10.1177/0363546508316021. [PubMed: 18443276]
15. Gabbe BJ, Bennell KL, Finch CF. Why are older Australian football players at greater risk of
hamstring injury? J Sci Med Sport. 2006; 9:327–333. http://dx.doi.org/10.1016/j.jsams.
2006.01.004. [PubMed: 16678486]
16. Gabbe BJ, Bennell KL, Finch CF, Wajswelner H, Orchard JW. Predictors of hamstring injury at
the elite level of Australian football. Scand J Med Sci Sports. 2006; 16:7–13. http://dx.doi.org/
10.1111/j.1600-0838.2005.00441.x. [PubMed: 16430675]
17. Gabbe BJ, Finch CF, Bennell KL, Wajswelner H. Risk factors for hamstring injuries in community
level Australian football. Br J Sports Med. 2005; 39:106–110. http://dx.doi.org/10.1136/bjsm.
2003.011197. [PubMed: 15665208]
18. Gibbs NJ, Cross TM, Cameron M, Houang MT. The accuracy of MRI in predicting recovery and
recurrence of acute grade one hamstring muscle strains within the same season in Australian Rules
football players. J Sci Med Sport. 2004; 7:248–258. [PubMed: 15362322]
19. Holm B, Kristensen MT, Bencke J, Husted H, Kehlet H, Bandholm T. Loss of knee-extension
strength is related to knee swelling after total knee arthroplasty. Arch Phys Med Rehabil. 2010;
91:1770–1776. http://dx.doi.org/10.1016/j.apmr.2010.07.229. [PubMed: 21044725]
20. Howell JN, Chleboun G, Conatser R. Muscle stiffness, strength loss, swelling and soreness
following exercise-induced injury in humans. J Physiol. 1993; 464:183–196. [PubMed: 8229798]
21. Jonhagen S, Nemeth G, Eriksson E. Hamstring injuries in sprinters. The role of concentric and
eccentric hamstring muscle strength and flexibility. Am J Sports Med. 1994; 22:262–266.
[PubMed: 8198197]
22. Koulouris G, Connell DA, Brukner P, Schneider-Kolsky M. Magnetic resonance imaging
parameters for assessing risk of recurrent hamstring injuries in elite athletes. Am J Sports Med.
2007; 35:1500–1506. http://dx.doi.org/10.1177/0363546507301258. [PubMed: 17426283]
23. Kraemer R, Knobloch K. A soccer-specific balance training program for hamstring muscle and
patellar and Achilles tendon injuries: an intervention study in premier league female soccer. Am J
Sports Med. 2009; 37:1384–1393. http://dx.doi.org/10.1177/0363546509333012. [PubMed:
19567665]
24. Orchard J, Best TM. The management of muscle strain injuries: an early return versus the risk of
recurrence. Clin J Sport Med. 2002; 12:3–5. [PubMed: 11854581]
25. Petersen J, Thorborg K, Nielsen MB, Budtz-Jørgensen E, Hölmich P. Preventive effect of eccentric
training on acute hamstring injuries in men’s soccer: a cluster-randomized controlled trial. Am J
SILDER et al. Page 16
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

Sports Med. 201139:2296–2303. http://dx.doi.org/10.1177/0363546511419277. [PubMed:
21825112]
26. Philippou A, Maridaki M, Bogdanis G, Halapas A, Koutsilieris M. Changes in the mechanical
properties of human quadriceps muscle after eccentric exercise. In Vivo. 2009; 23:859–865.
[PubMed: 19779124]
27. Proske U, Morgan DL, Brockett CL, Percival P. Identifying athletes at risk of hamstring strains
and how to protect them. Clin Exp Pharma-col Physiol. 2004; 31:546–550. http://dx.doi.org/
10.1111/j.1440-1681.2004.04028.x.
28. Schache A. Eccentric hamstring muscle training can prevent hamstring injuries in soccer players. J
Physiother. 2012; 58:58. http://dx.doi.org/10.1016/S1836-9553(12)70074-7. [PubMed: 22341384]
29. Schneider-Kolsky ME, Hoving JL, Warren P, Connell DA. A comparison between clinical
assessment and magnetic resonance imaging of acute hamstring injuries. Am J Sports Med. 2006;
34:1008–1015. http://dx.doi.org/10.1177/0363546505283835. [PubMed: 16476919]
30. Sherry MA, Best TM. A comparison of 2 rehabilitation programs in the treatment of acute
hamstring strains. J Orthop Sports Phys Ther. 2004; 34:116–125. http://dx.doi.org/10.2519/jospt.
2004.1062. [PubMed: 15089024]
31. Slavotinek JP, Verrall GM, Fon GT. Hamstring injury in athletes: using MR imaging
measurements to compare extent of muscle injury with amount of time lost from competition. AJR
Am J Roentgenol. 2002179:1621–1628. http://dx.doi.org/10.2214/ajr.179.6.1791621. [PubMed:
12438066]
32. Verrall GM, Kalairajah Y, Slavotinek JP, Spriggins AJ. Assessment of player performance
following return to sport after hamstring muscle strain injury. J Sci Med Sport. 2006; 9:87–90.
http://dx.doi.org/10.1016/j.jsams.2006.03.007. [PubMed: 16621702]
33. Verrall GM, Slavotinek JP, Barnes PG, Fon GT. Diagnostic and prognostic value of clinical
findings in 83 athletes with posterior thigh injury: comparison of clinical findings with magnetic
resonance imaging documentation of hamstring muscle strain. Am J Sports Med. 2003; 31:969–
973. [PubMed: 14623665]
34. Verrall GM, Slavotinek JP, Barnes PG, Fon GT, Esterman A. Assessment of physical examination
and magnetic resonance imaging findings of hamstring injury as predictors for recurrent injury. J
Orthop Sports Phys Ther. 2006; 36:215–224. http://dx.doi.org/10.2519/jospt.2006.2086. [PubMed:
16676871]
35. Warren P, Gabbe BJ, Schneider-Kolsky M, Bennell KL. Clinical predictors of time to return to
competition and of recurrence following hamstring strain in elite Australian footballers. Br J
Sports Med. 2010; 44:415–419. http://dx.doi.org/10.1136/bjsm.2008.048181. [PubMed:
18653619]
36. Woods C, Hawkins RD, Maltby S, Hulse M, Thomas A, Hodson A. The Football Association
Medical Research Programme: an audit of injuries in professional football—analysis of hamstring
injuries. Br J Sports Med. 2004; 38:36–41. http://dx.doi.org/10.1136/bjsm.2002.002352. [PubMed:
14751943]
SILDER et al. Page 17
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

KEY POINTS
FINDINGS: A modified PATS rehabilitation program and a PRES program
produced similar results with respect to muscle recovery and function following a
hamstring strain injury. Athletes participating in both rehabilitation groups continued
to show indication of injury on MRI following completion of rehabilitation, despite
meeting clinical clearance to return to sport.
IMPLICATIONS: The physical therapist should consider that hamstring muscle
recovery continues after an athlete meets clinical clearance to return to sport.
CAUTION: The relatively small sample size in this study limits any conclusions
regarding the effectiveness of either rehabilitation program at minimizing reinjury
risk.
SILDER et al. Page 18
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

FIGURE 1.
The percent cross-sectional area of injured muscle was estimated by considering all muscles
that exhibited T2 hyperintensity.
SILDER et al. Page 19
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

FIGURE 2.
Flow diagram outlining enrollment and testing procedures.
SILDER et al. Page 20
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

FIGURE 3.
Coronal and axial T2-weighted MRI scans taken after injury (A and B) and after completion
of rehabilitation (C and D). The tendon of the injured limb can initially appear wavy (A;
arrow). Scar tissue begins to form during the course of rehabilitation and is clearly visible on
MRI scans obtained after completion of rehabilitation (C and D; arrows). Edema and
hemorrhage (T2 hyperintensity) can extend into the fascial plane (A and B). Over the course
of time, fascial drainage can lengthen the craniocaudal extent of injury and result in MRI
measurements longer than the actual muscle/tendon damage. T2 hyperintensity was often
more concentrated during the initial MRI examination (A and B), compared to a more
diffuse signal present in the follow-up MRI examination (C and D). Abbreviation: MRI,
magnetic resonance imaging.
SILDER et al. Page 21
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

FIGURE 4.
Coronal and axial T2-weighted magnetic resonance images of subject 3, taken after initial
injury (A and B) and 7 days after reinjury (C and D). The location of reinjury was similar to
the initial injury. Early signs of scar tissue formation can be seen on the second set of
images (C and D; arrows).
SILDER et al. Page 22
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.
NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript

NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript
SILDER et al. Page 23
TABLE 1
Subject Characteristics
*
Program/Subject Gender, Age Method of Injury Muscles Involved, n Primary Muscle Primary Location
Distance
From Origin,
cm Return to
Sport, d Clinic Visits, n
Rehabilitation
Compliance
(Completed/
Assigned), d
PATS
4 Female, 16 y Sprinting 2 SM
†
Tendon 0.0 37 6 29/34
5 Male, 21 y Sprinting 1 BF Tendon 19.0 34 6 19/30
6 Male, 43 y Sprinting 1 BF
†
Mid-MTJ 12.4 33 4 20/27
11 Male, 18 y Sprinting 2 ST
†
Prox MTJ 0.0 28 5 12/13
12 Male, 25 y Sprinting 2 BF Prox MTJ 6.3 27 4 12/21
13 Female, 20 y Extreme stretch 0 NA
†
NA NA 23 4 13/17
14 Female, 18 y Cutting maneuver 1 SM Prox MTJ 21.2 23 5 16/20
15 Male, 46 y Sprinting 1 BF Tendon 17.3 23 2 14/20
16 Male, 40 y Sprinting 3 BF Mid-MTJ 12.6 23 4 18/20
18 Male, 20 y Sprinting 0 NA
†
NA NA 21 3 16/19
20 Male, 16 y Sprinting 2 ST Prox MTJ 8.5 20 3 12/12
23 Male, 21 y Extreme stretch 1 BF Distal MTJ 21.1 18 3 10/13
24 Female, 19 y Extreme stretch 0 NA
†
NA NA 17 3 12/13
27 Male, 36 y Sprinting 3 BF Mid-MTJ 5.2 Reinjury Reinjury NA
28 Male, 18 y Extreme stretch 2 BF Tendon 18.1 Dropout Dropout NA
29 Female, 30 y Sprinting 3 BF Tendon 0.0 Dropout Dropout NA
PRES
1 Male, 44 y Sprinting 2 BF
†
Prox MTJ 3.7 49 6 36/42
2 Male, 27 y Sprinting 4 BF Everywhere 4.4 47 7 35/40
3 Male, 17 y Sprinting 1 BF
†
Mid-MTJ 7.2 40 7 32/40
7 Male, 16 y Sprinting 2 BF Tendon 6.9 30 3 28/28
8 Male, 18 y Sprinting 2 BF Mid-MTJ 7.0 29 5 22/27
9 Male, 28 y Sprinting 1 BF Prox MTJ 8.4 28 4 19/24
10 Male, 28 y Sprinting 2 BF Mid-MTJ 13.8 28 3 18/21
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.

NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript
SILDER et al. Page 24
Program/Subject Gender, Age Method of Injury Muscles Involved, n Primary Muscle Primary Location
Distance
From Origin,
cm Return to
Sport, d Clinic Visits, n
Rehabilitation
Compliance
(Completed/
Assigned), d
17 Male, 17 y Sprinting 1 BF
†
Prox MTJ 0.0 23 4 12/13
19 Male, 16 y Sprinting 1 BF Mid-MTJ 17.5 20 3 17/17
21 Male, 17 y Sprinting 1 BF Prox MTJ 9.3 19 4 12/13
22 Male, 21 y Extreme stretch 1 SM Prox MTJ 5.5 19 2 11/13
25 Female, 22 y Cutting maneuver 1 SM Mid-MTJ 15.7 13 2 13/13
26 Male, 19 y Sprinting 2 BF Mid-MTJ 7.1 Reinjury Reinjury NA
Abbreviations: BF, biceps femoris; MRI, magnetic resonance imaging; MTJ, musculotendon junction; NA, not applicable; PATS, progressive agility and trunk stabilization; PRES, progressive running and
eccentric strengthening; Prox, proximal; SM, semimembranosus; ST, semitendinosus.
*
Subjects are numbered and sorted based on return-to-sport time (number of days from injury until being cleared to return to sport). Sixteen subjects participated in the PATS program and 13 subjects
participated in the PRES program. MRI was used to determine the number of muscles involved in the injury, the primary muscle injured, the primary location of injury, and the distance of injury from the
ischial tuberosity (distance of maximum T2 hyperintensity). Compliance of home rehabilitation was calculated as the ratio of completed home rehabilitation days (per self-report exercise log) divided by the
number of days assigned. NA represents no MRI indication of injury (ie, no T2 hyperintensity). No subject in this study experienced an injury to the distal aspect of the muscle; therefore, all injury locations
are relative to the proximal aspect of the muscle.
†
With respect to the muscle injured, the physical examination diagnosis and MRI disagreed in 9 subjects. No T2 hyperintensity was present in the initial MRI examination of 3 subjects. The muscles
injured, as determined from the initial physical examination, in these subjects were as follows: subject 13, ST and SM; subject 18, common insertion; subject 24, ST and SM. The muscles injured, as
determined on the initial physical examination, for the remaining 6 subjects were as follows: subject 1, ST and SM; subject 3, ST; subject 4, BF; subject 6, SM; subject 11, BF; subject 17, ST.
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.

NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript
SILDER et al. Page 25
TABLE 2
Summary of MRI Measures Conducted Before and After Completion of Rehabilitation
*
Program/Subject
Craniocaudal Length, cm Cross-sectional Area, % Normalized Maximum T2 Hyperintensity
Initial Final Initial Final Initial Final
PATS
4 3.2 0.0 100 0 1.5 1.2
5 9.3 7.3 25 37 1.9 1.6
6 18.8 5.5 79 1 2.5 1.7
11 23.7 22.8 71 55 4.6 3.4
12 17.1 6.9 20 2 3.3 2.0
13 NA NA NA NA NA NA
14 7.7 2.5 47 6 3.4 2.0
15 16.6 6.8 36 14 3.5 2.2
16 25.2 23.5 79 55 3.5 2.9
18 NA NA NA NA NA NA
20 12.8 3.6 33 12 4.2 1.4
23 12.2 4.8 40 43 2.6 2.5
24 NA NA NA NA NA NA
27 33.1 Reinjury 100 Reinjury 1.5 Reinjury
28 19.3 Dropout 86 Dropout 3.3 Dropout
29 13.6 Dropout 100 Dropout 4.1 Dropout
PRES
1 15.6 11.4 64 22 2.4 2.1
2 35.5 28.6 48 28 2.9 3.3
3 18.7 Reinjury 98 Reinjury 3.3 Reinjury
7 30.4 27.8 55 16 2.1 2.1
8 23.5 23.1 61 33 1.8 1.5
9 15.5 12.5 100 40 3 2.6
10 8.7 8.6 35 13 3.4 2.5
17 24.1 22.8 91 100 2.8 2.4
19 7.9 10.4 16 30 3 2.8
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.

NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript
SILDER et al. Page 26
Program/Subject
Craniocaudal Length, cm Cross-sectional Area, % Normalized Maximum T2 Hyperintensity
Initial Final Initial Final Initial Final
21 13.1 14.6 100 25 2.4 1.6
22 5.2 12.3 70 43 4.6 2.2
25 8.7 2.3 9 2 2.1 1.9
26 6.8 Reinjury 58 Reinjury 2.9 Reinjury
Abbreviations: MRI, magnetic resonance imaging; NA, not applicable; PATS, progressive agility and trunk stabilization; PRES, progressive running and eccentric strengthening.
*
MRI was used to determine the craniocaudal length of injury, percent cross-sectional area, and normalized maximum T2 hyperintensity after injury and after completion of rehabilitation. Because more
than 1 muscle is often injured,10,31,33 craniocaudal length and percent cross-sectional area were measured with respect to the total injured area. NA represents no magnetic resonance imaging indication of
injury (no T2 hyperintensity).
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.

NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript
SILDER et al. Page 27
TABLE 3
Summary of Physical Examination Results Conducted Before and After Completion of Rehabilitation
*
PATS†PRES‡
Noninjured Injured Reported Pain, n Noninjured Injured Reported Pain, n
Initial evaluation
Hip extension strength
§
Knee flexed 5 (4+ to 5) 4 (2 to 5) 7 5 (4+ to 5) 4+ (3 to 5) 5
Knee extended 5 (4+ to 5) 4 (2 to 5) 9 5 (4+ to 5) 4 (3 to 5) 8
Knee flexion strength
§
Knee flexed to 15° 5 (5) 4+ (3 to 4+) 10 5 (5) 4+ (3+ to 5) 11
Knee flexed to 90° 5 (5) 4 (3+ to 4+) 10 5 (5) 4 (4+ to 5) 10
Knee flexed to 90° with IR 5 (5) 4 (3 to 5) 8 5 (5) 4 (3 to 5) 7
Knee flexed to 90° with ER 5 (5) 4 (4+ to 5) 5 5 (5) 4 (3+ to 5) 7
Straight leg raise, deg
║
81 ± 14 63 ± 18 … 80 ± 15 70 ± 16 …
Active knee extension, deg 23 ± 10 21 ± 21 … 29 ± 12 26 ± 9 …
Passive knee extension, deg
║
34 ± 17 34 ± 20 … 39 ± 22 35 ± 21 …
Length of pain with palpation, cm
║
0.0 9.9 ± 5.2 … 0.0 8.3 ± 3.0 …
Final evaluation
Hip extension strength
§
Knee flexed 5 (4+ to 5) 5 (4+ to 5) 0 5 (5) 5 (4+ to 5) 1
Knee extended 5 (4+ to 5) 5 (4+ to 5) 0 5 (5) 5 (4+ to 5) 0
Knee flexion strength
§
Knee flexed to 15° 5 (5) 5 (4 to 5) 1 5 (5) 5 (5) 0
Knee flexed to 90° 5 (5) 5 (4+ to 5) 0 5 (5) 5 (4+ to 5) 1
Knee flexed to 90° with IR 5 (5) 5 (4 to 5) 1 5 (5) 5 (4+ to 5) 1
Knee flexed to 90° with ER 5 (5) 5 (5) 0 5 (5) 5 (5) 0
Straight leg raise, deg
║
86 ± 14 83 ± 13 … 78 ± 13 80 ± 13 …
Active knee extension, deg
║
18 ± 8 18 ± 10 … 26 ± 12 23 ± 11 …
Passive knee extension, deg
║
13 ± 9 13 ± 9 … 21 ± 11 18 ± 9 …
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.

NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript
SILDER et al. Page 28
PATS†PRES‡
Noninjured Injured Reported Pain, n Noninjured Injured Reported Pain, n
Length of pain with palpation, cm
║
0.0 0.0 … 0.0 0.0 …
Abbreviations: ER, external rotation; IR, internal rotation; PATS, progressive agility and trunk stabilization; PRES, progressive running and eccentric strengthening.
*
Two of the original 29 subjects dropped out of the study and 2 subjects sustained a reinjury prior to completion of rehabilitation.
†
At initial evaluation, n = 16; at final evaluation, n = 13.
‡
At initial evaluation, n = 13; at final evaluation, n = 11.
§
Values are median (range of scores reported), with a 5-point maximum. Isometric strength tests were done using a standard manual muscle testing grading scale. For each strength test, the number of
subjects who reported pain in their injured limb is indicated.
║
Values are mean ± SD.
J Orthop Sports Phys Ther
. Author manuscript; available in PMC 2013 August 23.